This invention relates to systems for measuring the relative position of first and second objects movable relative to one another, of the type comprising at least one measuring graduation carrier connected to the first object, a measuring incremental graduation defined by the measuring graduation carrier, at least one measuring scanning unit connected to the second object to scan the measuring incremental graduation, and means for defining at least one reference position at a selected absolute position with respect to the measuring incremental graduation.
In such position measuring systems it is a known practice in order to determine a reference position to drive either the machine components or the measuring system components which are movable relative to one another out of a starting position up to a reference mark. Once properly positioned, a position value appropriate for the reference mark is determined and stored, or alternately the reference mark is used to identify a zero position. In this way, an incremental measuring system can be calibrated as to absolute position.
Such a calibration process is possible with incremental length or angle measuring systems such as that described in German Patent DE-PS No. 19 64 381. This calibration process, however, requires unimpeded relative movability of the two objects to be measured, since the components of the measuring system are firmly joined with these objects and thus these objects must be shifted in common with the measuring system components to the reference mark.
German DE-OS No. 16 73 887 describes a measuring system for a machine, which permits a reference position to be determined in the event that the machine slide piece is clamped fast to the machine bed. First the slide piece must be driven into the position that is later to be declared the reference position for zero. Then the slide piece is clamped fast to the machine bed. Thereafter the scanning plate of the measuring arrangement is driven relative to the scale until the reference mark is reached. On reaching the reference mark the electronic counter of the measuring system is set to the value zero. The clamping for the machine slide piece can then be released and the slide piece can be driven to the desired position. The position of the reference mark therefore defines the reference position for further operating cycles.
This prior art process for determining a reference position is utilized before the working processes proper are begun. This approach is no longer suitable however in the event working operations have already begun, as for example when operations in progress are interrupted and the reference position must be regained. Such an interruption of a running operation can occur through a power failure for example. In the case of a manipulating automatic unit such as is generally referred to as an industrial robot, in the event of a power failure the robot remains standing in its momentary position. The measuring value referred to the original reference position is lost however due to the power failure, and the measuring operation is interrupted. In order to continue the interrupted working process, the reference position must be regained. A return movement of the robot out of its momentary position into the original starting position is as a general rule not possible. This is because a tool being manipulated by the robot may for example be in engagement with a work piece at the time of power interruption.